Sugar is the great fuel substance of the body. About two-thirds or our food ordinarily consists of starchy materials which are digested into sugar. When we add to this the sugarlike remains of all the protein food which is not actually used for the repair of tissues we see that this substance makes up the great bulk of the material which is carried by the blood to the tissues. This material is handled in the body in an interesting way which depends on the curious fact that although sugar is the most important fuel for living cells they cannot endure its presence in them or in the fluids surrounding them except in very small amounts. Sugar, as we all know, is very soluble in water, and it would be perfectly possible for the blood to dissolve all the sugar that enters it from the digestive tract and simply carry it in solution until the tissues withdrew it for their needs; but this would mean that immediately after every meal the percentage of sugar in the blood would mount up to a high point from which it would gradually sink as the sugar was taken out, to mount again after the next meal when absorption began again. This does not actually happen because it is prevented by the liver, which has as the most important of its many functions that of storing the sugar that is taken up by the blood from the digestive tract and dealing it out little by little as the cells of the body need it. Back in the chapter on digestion we saw that all the blood that passes through the intestinal tract is gathered up by the portal vein and passes through the capillaries of the liver. It is during this passage through the liver that the sugar is taken out of the blood and stored in the form of a less easily dissolved material known as animal starch or _glycogen_. The liver cells have the ability to convert sugar into glycogen and they do this whenever the amount of sugar in the blood passing through them is greater than the very small amount which is suitable for the body cells. The blood that leaves the liver carries in it only this small percentage. The liver cells have the ability to change glycogen back into sugar, and this they do whenever the blood that enters them is deficient in it, so that the blood leaving the liver tends always to have the same amount of sugar in it. Whenever it enters with more, there is a conversion of sugar into glycogen; whenever it enters with less, there is a conversion of glycogen back into sugar.